As Gmail manager allows multiple email check option,you can also view mail snippets which flashes as a small window showing 30+ odd unread messages without the need of opening Gmail website.You can also switch between your different Gmail accounts with a single click.

The installation of the addon is pretty simple,download Gmail manager and restart the firefox browser.The addon will pop up a window where you can add your Gmail username and passords,alternately you can import/export Gmail login details.A small icon appears in the status bar reflecting number of spam mails and unread messages in your inbox!